How Human Brains Compare To Animal Brains

Abstract

The encephalon is the about complex organ that ever evolved. The brain controls important functions in the body like keeping your heartbeat and your animate normal. It controls the motility of your eyes across the page equally you read this, it makes sense of the ink on the page to form words, and it links these words with concepts in your memory and makes new concepts as y'all learn. And the brain was as well the function of your body that made the conclusion to read this article in the first place. In different animals, brains are very unlike. There are big and small-scale brains, smooth and wrinkled brains, brains with some parts larger than others. How are these brains built? How did they come up to exist so unlike from each other—and why does that matter?

Animals Share an Evolutionary History

Not only humans take brains, of form. Almost all animals have a nervous system of some kind (only sponges do not). The brains of different animals are dissimilar in some ways merely similar in many other ways. This is considering all life on the planet shares a history: all animals evolved from common ancestors, and then they inherited some of the characteristics from these ancestors. This is similar a brother and a sis who are alike because they have the same parents, first cousins who take the same grandparents, 2d cousins who share smashing-grandparents, and and so on. All life shares a neat-great-peachy-grandparent in the distant by.

We tin use these similarities and differences betwixt animals to put the animals together into groups. Ii birds—say, an eagle and a parrot—accept more in common than an eagle and a monkey. And they are closer relatives. In a family, this would be like maxim that a brother is more like his sister than he is like his cousin. A blood brother and a sis have the same parents, just with their cousin they simply share the same grandparents, farther dorsum. Still, all mammals have fur and produce milk to feed their young. Every bird has feathers and lays eggs. And these similarities between shut relatives appear in their brains, too.

Each species has their different habits: a monkey jumps from branch to co-operative until information technology finds some fruit to eat, a bat flies effectually in the dark of the dark between trees, a whale swims in the wide open up ocean. Since the brains of these animals help them to do all these tasks, we would judge that their brains would exist very different. Only it turns out that the main parts of the brain and the connections within the brain are all pretty much the same for all mammals. This similarity in brain structure exists because of the evolutionary history that all these brains share.

In fact, if we put brains of different mammals next to each other, the similarities are easy to spot. Even though the brains vary (a lot) in their size and in their folds, they all take the same parts. All these brains take a cerebral cortex, a cerebellum, and a brain stalk (see Figure 1B). Also, the same kinds of cells brand up all brains: they are made of neurons, glial cells, and the cells that make the capillaries (small blood vessels) that bring claret into the brain (Figure 1A). Neurons transmit data to other neurons through their branches, across connections called synapses. Glial cells are of iii kinds. Microglia are the allowed organization in the encephalon. Oligodendrocytes wrap the branches of the neurons and make information movement faster from 1 neuron to the next. Astrocytes do a lot of things, keeping everything in gild—from helping neurons make synapses to providing them with nutrients.

• Effigy 1
• A. All brains accept ii connected sides, chosen the right and left hemispheres. In the figure, you lot see the inside of the right hemisphere of a man brain. The principal colored parts are the cognitive cortex, the cerebellum, and the brainstem. The cerebral cortex, the "thinking office" of the brain, is where things similar speech communication, planning, emotions, and problem-solving accept place. Depending on the species, the cerebral cortex can be smooth or full of folds (similar ours, in the figure). The cerebellum looks similar a small brain and also has folds. Keeping proper posture, balance and doing delicate, careful movements are all tasks involving the cerebellum. Then are thinking and handling emotions. The brainstem is the region that connects the whole brain with the body, through several nerves and the spinal string, and controls vital automatic movements such as breathing, digestion, center rate, and blood pressure. B. Neurons (in aureate)—they receive, process, and send information to other neurons. Axons (the "cables" from one neuron to another) are wrapped by oligodendrocyte cells (bluish), which create a blanket, much like the rubber around electric wires, that insulates axons and helps signals be conducted from one neuron to the side by side more quickly. Astrocytes (red) have lots of functions. Scientists thought, at get-go, that all astrocytes did was make full in the space betwixt neurons, property them apart and giving them something like a "skeleton" on the exterior. But now, we know that astrocytes do much more: they feed neurons, control the formation of the contacts betwixt neurons (the synapses) and how they function, control the concentration of many substances in the space outside the cells, and repair injuries. Finally, microglia (purple) observe and destroy strange cells and particles that do not belong in the brain, protecting it, similar the immune system protects the residuum of the body.

Is There Just One Rule to Build Brains?

Even though the parts are the same, this does non mean that brains of the same size are built of the aforementioned amounts of each kind of jail cell. It is too not truthful that a bigger brain is always made of more than cells than a smaller brain.

We can remember about this with an example. Imagine that you receive two brains of the same weight but belonging to different species. This is what nosotros see in Effigy 2: a rhesus monkey brain and a capybara brain (a capybara is the largest rodent live, it looks like a giant guinea pig). Both brains weigh about lxxx yard. Y'all would probably say the brains accept the same number of neurons—and so would many scientists. Until about 10 years ago, most researchers expected brains of the aforementioned size to have the aforementioned number of neurons. They thought that at that place was only one "recipe" in nature for edifice brains, and that all brains were made the same manner. That also meant that the bigger the brain, the more neurons it would take.

• Figure 2 - Nature has different ways of calculation neurons to primate and rodent brains as they change in size from one species to another.
• This ways that when a rodent brain gets larger, information technology does not gain many neurons. In dissimilarity, when a primate brain gets bigger, its number of neurons grows more or less in proportion to how much larger the brain is.

We now know that neither of these things is true. In 2005, one of us (Suzana) adult a new method that lets us count how many neurons brand up a brain [1]. This method dissolves the brain into soup (Box 1), starting with a recently dead brain that has been treated with a chemical called paraformaldehyde (PFA) to brand the cells more resistant (if the brain is not treated with PFA, you could damage information technology just by touching it). If yous do not use the soup method and instead accept a slice of a brain and count how many cells are there, you might count cells from a place in the encephalon that is full of neurons and then you would call back that the whole encephalon has a lot of neurons. Just the number of cells is non the same in all parts of the encephalon—some encephalon parts have more neurons, some take fewer. That is why we make encephalon soup: because and then, all drops of "soup" have near the same number of cells one time you stir the soup actually well. Besides, since this soup contains each and every cell that fabricated up the original part of the brain, or perhaps the whole brain (if yous did not separate its parts first), then past looking under the microscope and counting the cells in just a niggling chip of the "soup" nosotros can get a good idea of how many cells the whole brain has.

Box 1. How to brand brain soup

Here is the idea: nosotros want to take a encephalon and destroy everything to make soup—except the nuclei of the cells that make up the brain. Because each brain prison cell has only one nuclei, if nosotros find the number of nuclei, we find the number of cells that brand upwards the brain.

Step i: Put the brain in paraformaldehyde (PFA) for a few days. This process is called fixation. PFA will make the nuclei membranes stronger, then they won't break afterwards.

Step 2: Split the part of the brain that yous want to know about. Then slice it into small pieces.

Stride 3: Put the sliced brain tissue inside the "soup-maker glass." This tool is fabricated of 2 pieces of glass, carefully built and so that when one is inside the other the space betwixt the glasses is very very small (search for "glass tissue grinder" on the web to run across pictures).

Step 4: Add together a detergent to help break up the tissue. Apply a special detergent that won't destroy the nuclear membrane (call back, we want them preserved so we can count the nuclei in the soup).

Pace 5: Motility the inside glass up and down and rotate it, to break up the brain tissue. This is chosen fractionating. By rubbing these two pieces of the glass against each other, the friction breaks the brain pieces into smaller and smaller pieces, eventually leaving only the nuclei of the cells (which is what nosotros count on the microscope). This is sort of like making juice: you printing the encephalon confronting the glass, simply like a fruit is pressed against the rotating juicer reamer to excerpt its juice.

Permit united states have imaginary brains in our easily again, this time ii primate brains, one twice the size of the other. How many neurons would the larger one have? Well, in this case, near twice the number of neurons of the smaller encephalon. At present, if brains were all built in the same way, with the same uniform recipes, we would see the same thing in rodents. Then, if we took at present two rodent brains, one brain twice every bit big as the other, we would look the larger ane to have twice every bit many neurons as the smaller ane. Simply if you lot practise this with actual brains you will see that the larger rodent encephalon has fewer neurons than expected—less than twice as many neurons in a brain that is twice as large. This means rodents and primates have different rules for building brains. Most importantly, if you were to compare a largish primate brain (like the brain of a monkey) with a similarly largish rodent brain (like the encephalon of a capybara), yous would notice that the monkey brain has many more neurons than the rodent brain.

The dissimilar relationships between encephalon size and number of neurons for rodents and primates means that different recipes, or "rules," utilise when these brains are built in nature. The dominion for primates says "if you lot have ten times as many neurons, you go a brain about 10 times equally large." The rule for rodents says "if yous have 10 times as many neurons, you get a brain that is 45 times larger." The different rules mean that rodent brains abound much faster than primate brains as they proceeds neurons. And so, without knowing the rules are different, y'all would estimate that a primate encephalon has fewer neurons than it actually has. Simply the truth is, there are many more neurons in a primate brain than in a rodent brain of the same size. To see this with real numbers for real brains, yous can look at the examples in Figure 2.

Testing and Changing Rules

If y'all know the rule relating brain size to the number of neurons, you lot can predict how many neurons a brain of a certain size would take, co-ordinate to that rule. The rules will as well show what limits be for building brains.

To pick a familiar example, a generic primate brain of 1.5 kg, just like our encephalon, should have 93 billion neurons, according to the primate rule. Our best estimate for how many neurons nosotros accept is 86 billion neurons, on average [two]. That is pretty close. What this tells us is that, well, nosotros have a primate encephalon: we are that generic primate. In other words, as far as numbers of neurons go, our brains are not special compared to those of our closest relatives, the apes and monkeys. We have just as many neurons as a primate with our brain size should have. Just considering nosotros are the primate with the biggest brain, we have the most neurons of any primate.

But what would things be like if nosotros were rodents? What if, instead of a primate brain of one.5 kg, nosotros had a rodent brain of 1.5 kg? We tin can do the math to observe out: the brain would take only 19 billion neurons, far fewer than our 86 billion—and the encephalon would probably exist able to do much less. Now let usa invert the question: how big would a rodent brain have to be to have 86 billion neurons like ours? Well, actually big. Information technology would counterbalance over 30 kg! This imaginary rodent would need a body of over lxxx t to carry a brain this large—this is every bit much equally 20 elephants together, every bit shown in Figure 3! Such a giant rodent could never exist: its encephalon would be crushed under its ain weight (retrieve that the brain does not have a structure inside itself to support it, like a hard skeleton). No wonder such a huge rodent has never appeared in the history of life.

• Figure three - If at that place were a rodent with every bit many neurons as a man has—86 billion—its brain would counterbalance over 30 kg.
• This means that this hypothetical rodent would demand a body of 80 t to behave the super heavy brain. To become an idea of what this super rodent would have to look similar, imagine putting it on a scale: it would weigh the aforementioned as 20 elephants!

Other types of mammals studied and so far—bats, insectivores , carnivores , marsupials , and cetaceans —take a lot in mutual, but also many differences. It seems that nature has many different recipes, or rules, for building brains. There is a lot of multifariousness in brains, only similar different types of animals look dissimilar on the exterior. But primates are the ones that fit the about neurons in the cerebral cortex, the "thinking" role of the brain, compared with other species that have similar size brains. This does not mean everything is different in primate brains. Remember that all animals share common grand-chiliad-grand-grandparents from many years back, and some brain rules are shared. Simply one quick example: the dominion relating the number of glial cells (call up them? Run across Figure ane) to brain size is the same for all groups of mammals. Because this is common to all mammals, we remember it must exist very of import in order for the brain to work well.

All of information nigh brain rules is important for many reasons. The human encephalon is not the largest of all brains. For instance, the largest state mammal, the elephant, has a cerebral cortex that weighs twice equally much every bit the human cortex. However, because the brains of different animals are built in unlike ways, the elephant's gigantic cortex has only a third of the neurons that humans have in their cerebral cortex [3, iv]. The lesson here is that size may be important, simply it is far from everything when it comes to comparison brains. Because of the different "recipes" that nature uses to build brains, comparing primate brains to rodent brains is like comparing apples to oranges. Besides, if we want to know about the abilities of an animal, the number of neurons should matter much more the size of the brain. That is because neurons are the cells that procedure all the information the brain gets and turns information technology into action. So having more neurons is probably better, for brain ability, at least—equally long as the brain does non become likewise big!

Glossary

Rodent: ↑ Group of small mammals, known for their large front teeth. Mice and rats are rodents.

Primate: ↑ The group of animals made upwardly of humans, all apes and monkeys, from minor lemurs to the largest, the gorilla.

Insectivores: ↑ A group of small mammals which eat insects. These are hedgehogs and moles. Also, the smallest mammal in the world—the Etruscan shrew, weighing 2 g—belongs to this grouping.

Carnivores: ↑ These are mammals known for hunting and eating meat. Dogs and cats fit hither, but also lions, bears, and hyenas.

Marsupials: ↑ These are the mammals with a pouch to carry their young. Most of them are from Commonwealth of australia. Kangaroos, opossums, and koalas are marsupials.

Cetaceans: ↑ Aquatic mammals, almost all living in the sea. Whales, dolphins, and porpoises.

Disharmonize of Involvement Argument

The authors declare that the research was conducted in the absence of any commercial or fiscal relationships that could be construed as a potential conflict of interest.

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References

[ane] ↑ Herculano-Houzel, Southward., and Lent, R. 2005. Isotropic fractionator: a simple, rapid method for the quantification of total jail cell and neuron numbers in the brain. J. Neurosci. 25(10):2518–21. doi:10.1523/JNEUROSCI.4526-04.2005

[2] ↑ Azevedo, F. A., Carvalho, L. R., Grinberg, L. T., Farfel, J. M., Ferretti, R. E., Leite, R. Eastward., et al. 2009. Equal numbers of neuronal and nonneuronal cells brand the human brain an isometrically scaled-upwards primate brain. J. Comp. Neurol. 513(5):532–41. doi:10.1002/cne.21974

[iii] ↑ Herculano-Houzel, S., Manger, P. R., and Kaas, J. H. 2014. Encephalon scaling in mammalian evolution as a consequence of concerted and mosaic changes in numbers of neurons and boilerplate neuronal cell size. Front. Neuroanat. 8:77. doi:10.3389/fnana.2014.00077

[four] ↑ Herculano-Houzel, S., Avelino-de-Souza, K., Neves, Yard., Porfírio, J., Messeder, D., Mattos Feijó, L., et al. 2014. The elephant brain in numbers. Forepart. Neuroanat. 8:46. doi:10.3389/fnana.2014.00046

Source: https://kids.frontiersin.org/articles/10.3389/frym.2017.00021

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